Documentation/virtual/kvm/nested-vmx.txt

   1 Nested VMX
   2 ==========
   3
   4 Overview
   5 ---------
   6
   7 On Intel processors, KVM uses Intel's VMX (Virtual-Machine eXtensions)
   8 to easily and efficiently run guest operating systems. Normally, these guests
   9 *cannot* themselves be hypervisors running their own guests, because in VMX,
  10 guests cannot use VMX instructions.
  11
  12 The "Nested VMX" feature adds this missing capability - of running guest
  13 hypervisors (which use VMX) with their own nested guests. It does so by
  14 allowing a guest to use VMX instructions, and correctly and efficiently
  15 emulating them using the single level of VMX available in the hardware.
  16
  17 We describe in much greater detail the theory behind the nested VMX feature,
  18 its implementation and its performance characteristics, in the OSDI 2010 paper
  19 "The Turtles Project: Design and Implementation of Nested Virtualization",
  20 available at:
  21
  22         http://www.usenix.org/events/osdi10/tech/full_papers/Ben-Yehuda.pdf
  23
  24
  25 Terminology
  26 -----------
  27
  28 Single-level virtualization has two levels - the host (KVM) and the guests.
  29 In nested virtualization, we have three levels: The host (KVM), which we call
  30 L0, the guest hypervisor, which we call L1, and its nested guest, which we
  31 call L2.
  32
  33
  34 Known limitations
  35 -----------------
  36
  37 The current code supports running Linux guests under KVM guests.
  38 Only 64-bit guest hypervisors are supported.
  39
  40 Additional patches for running Windows under guest KVM, and Linux under
  41 guest VMware server, and support for nested EPT, are currently running in
  42 the lab, and will be sent as follow-on patchsets.
  43
  44
  45 Running nested VMX
  46 ------------------
  47
  48 The nested VMX feature is disabled by default. It can be enabled by giving
  49 the "nested=1" option to the kvm-intel module.
  50
  51 No modifications are required to user space (qemu). However, qemu's default
  52 emulated CPU type (qemu64) does not list the "VMX" CPU feature, so it must be
  53 explicitly enabled, by giving qemu one of the following options:
  54
  55      -cpu host              (emulated CPU has all features of the real CPU)
  56
  57      -cpu qemu64,+vmx       (add just the vmx feature to a named CPU type)
  58
  59
  60 ABIs
  61 ----
  62
  63 Nested VMX aims to present a standard and (eventually) fully-functional VMX
  64 implementation for the a guest hypervisor to use. As such, the official
  65 specification of the ABI that it provides is Intel's VMX specification,
  66 namely volume 3B of their "Intel 64 and IA-32 Architectures Software
  67 Developer's Manual". Not all of VMX's features are currently fully supported,
  68 but the goal is to eventually support them all, starting with the VMX features
  69 which are used in practice by popular hypervisors (KVM and others).
  70
  71 As a VMX implementation, nested VMX presents a VMCS structure to L1.
  72 As mandated by the spec, other than the two fields revision_id and abort,
  73 this structure is *opaque* to its user, who is not supposed to know or care
  74 about its internal structure. Rather, the structure is accessed through the
  75 VMREAD and VMWRITE instructions.
  76 Still, for debugging purposes, KVM developers might be interested to know the
  77 internals of this structure; This is struct vmcs12 from arch/x86/kvm/vmx.c.
  78
  79 The name "vmcs12" refers to the VMCS that L1 builds for L2. In the code we
  80 also have "vmcs01", the VMCS that L0 built for L1, and "vmcs02" is the VMCS
  81 which L0 builds to actually run L2 - how this is done is explained in the
  82 aforementioned paper.
  83
  84 For convenience, we repeat the content of struct vmcs12 here. If the internals
  85 of this structure changes, this can break live migration across KVM versions.
  86 VMCS12_REVISION (from vmx.c) should be changed if struct vmcs12 or its inner
  87 struct shadow_vmcs is ever changed.
  88
  89         typedef u64 natural_width;
  90         struct __packed vmcs12 {
  91                 /* According to the Intel spec, a VMCS region must start with
  92                  * these two user-visible fields */
  93                 u32 revision_id;
  94                 u32 abort;
  95
  96                 u32 launch_state; /* set to 0 by VMCLEAR, to 1 by VMLAUNCH */
  97                 u32 padding[7]; /* room for future expansion */
  98
  99                 u64 io_bitmap_a;
 100                 u64 io_bitmap_b;
 101                 u64 msr_bitmap;
 102                 u64 vm_exit_msr_store_addr;
 103                 u64 vm_exit_msr_load_addr;
 104                 u64 vm_entry_msr_load_addr;
 105                 u64 tsc_offset;
 106                 u64 virtual_apic_page_addr;
 107                 u64 apic_access_addr;
 108                 u64 ept_pointer;
 109                 u64 guest_physical_address;
 110                 u64 vmcs_link_pointer;
 111                 u64 guest_ia32_debugctl;
 112                 u64 guest_ia32_pat;
 113                 u64 guest_ia32_efer;
 114                 u64 guest_pdptr0;
 115                 u64 guest_pdptr1;
 116                 u64 guest_pdptr2;
 117                 u64 guest_pdptr3;
 118                 u64 host_ia32_pat;
 119                 u64 host_ia32_efer;
 120                 u64 padding64[8]; /* room for future expansion */
 121                 natural_width cr0_guest_host_mask;
 122                 natural_width cr4_guest_host_mask;
 123                 natural_width cr0_read_shadow;
 124                 natural_width cr4_read_shadow;
 125                 natural_width cr3_target_value0;
 126                 natural_width cr3_target_value1;
 127                 natural_width cr3_target_value2;
 128                 natural_width cr3_target_value3;
 129                 natural_width exit_qualification;
 130                 natural_width guest_linear_address;
 131                 natural_width guest_cr0;
 132                 natural_width guest_cr3;
 133                 natural_width guest_cr4;
 134                 natural_width guest_es_base;
 135                 natural_width guest_cs_base;
 136                 natural_width guest_ss_base;
 137                 natural_width guest_ds_base;
 138                 natural_width guest_fs_base;
 139                 natural_width guest_gs_base;
 140                 natural_width guest_ldtr_base;
 141                 natural_width guest_tr_base;
 142                 natural_width guest_gdtr_base;
 143                 natural_width guest_idtr_base;
 144                 natural_width guest_dr7;
 145                 natural_width guest_rsp;
 146                 natural_width guest_rip;
 147                 natural_width guest_rflags;
 148                 natural_width guest_pending_dbg_exceptions;
 149                 natural_width guest_sysenter_esp;
 150                 natural_width guest_sysenter_eip;
 151                 natural_width host_cr0;
 152                 natural_width host_cr3;
 153                 natural_width host_cr4;
 154                 natural_width host_fs_base;
 155                 natural_width host_gs_base;
 156                 natural_width host_tr_base;
 157                 natural_width host_gdtr_base;
 158                 natural_width host_idtr_base;
 159                 natural_width host_ia32_sysenter_esp;
 160                 natural_width host_ia32_sysenter_eip;
 161                 natural_width host_rsp;
 162                 natural_width host_rip;
 163                 natural_width paddingl[8]; /* room for future expansion */
 164                 u32 pin_based_vm_exec_control;
 165                 u32 cpu_based_vm_exec_control;
 166                 u32 exception_bitmap;
 167                 u32 page_fault_error_code_mask;
 168                 u32 page_fault_error_code_match;
 169                 u32 cr3_target_count;
 170                 u32 vm_exit_controls;
 171                 u32 vm_exit_msr_store_count;
 172                 u32 vm_exit_msr_load_count;
 173                 u32 vm_entry_controls;
 174                 u32 vm_entry_msr_load_count;
 175                 u32 vm_entry_intr_info_field;
 176                 u32 vm_entry_exception_error_code;
 177                 u32 vm_entry_instruction_len;
 178                 u32 tpr_threshold;
 179                 u32 secondary_vm_exec_control;
 180                 u32 vm_instruction_error;
 181                 u32 vm_exit_reason;
 182                 u32 vm_exit_intr_info;
 183                 u32 vm_exit_intr_error_code;
 184                 u32 idt_vectoring_info_field;
 185                 u32 idt_vectoring_error_code;
 186                 u32 vm_exit_instruction_len;
 187                 u32 vmx_instruction_info;
 188                 u32 guest_es_limit;
 189                 u32 guest_cs_limit;
 190                 u32 guest_ss_limit;
 191                 u32 guest_ds_limit;
 192                 u32 guest_fs_limit;
 193                 u32 guest_gs_limit;
 194                 u32 guest_ldtr_limit;
 195                 u32 guest_tr_limit;
 196                 u32 guest_gdtr_limit;
 197                 u32 guest_idtr_limit;
 198                 u32 guest_es_ar_bytes;
 199                 u32 guest_cs_ar_bytes;
 200                 u32 guest_ss_ar_bytes;
 201                 u32 guest_ds_ar_bytes;
 202                 u32 guest_fs_ar_bytes;
 203                 u32 guest_gs_ar_bytes;
 204                 u32 guest_ldtr_ar_bytes;
 205                 u32 guest_tr_ar_bytes;
 206                 u32 guest_interruptibility_info;
 207                 u32 guest_activity_state;
 208                 u32 guest_sysenter_cs;
 209                 u32 host_ia32_sysenter_cs;
 210                 u32 padding32[8]; /* room for future expansion */
 211                 u16 virtual_processor_id;
 212                 u16 guest_es_selector;
 213                 u16 guest_cs_selector;
 214                 u16 guest_ss_selector;
 215                 u16 guest_ds_selector;
 216                 u16 guest_fs_selector;
 217                 u16 guest_gs_selector;
 218                 u16 guest_ldtr_selector;
 219                 u16 guest_tr_selector;
 220                 u16 host_es_selector;
 221                 u16 host_cs_selector;
 222                 u16 host_ss_selector;
 223                 u16 host_ds_selector;
 224                 u16 host_fs_selector;
 225                 u16 host_gs_selector;
 226                 u16 host_tr_selector;
 227         };
 228
 229
 230 Authors
 231 -------
 232
 233 These patches were written by:
 234      Abel Gordon, abelg <at> il.ibm.com
 235      Nadav Har'El, nyh <at> il.ibm.com
 236      Orit Wasserman, oritw <at> il.ibm.com
 237      Ben-Ami Yassor, benami <at> il.ibm.com
 238      Muli Ben-Yehuda, muli <at> il.ibm.com
 239
 240 With contributions by:
 241      Anthony Liguori, aliguori <at> us.ibm.com
 242      Mike Day, mdday <at> us.ibm.com
 243      Michael Factor, factor <at> il.ibm.com
 244      Zvi Dubitzky, dubi <at> il.ibm.com
 245
 246 And valuable reviews by:
 247      Avi Kivity, avi <at> redhat.com
 248      Gleb Natapov, gleb <at> redhat.com
 249      Marcelo Tosatti, mtosatti <at> redhat.com
 250      Kevin Tian, kevin.tian <at> intel.com
 251      and others.